In the early books of the Iliad, Achilles refuses to fight because he is angry that Agamemnon, the Greek leader, has taken the lovely Briseis as his own, since she was promised to Achilles and is rightfully his. However, Achilles does allow his best friend Patroclus to fight while wearing his armor. This will intimidate the Trojans, as once they see the armor, they will believe the mighty Achilles has reentered the battlefield.
Even though he is wearing the armor of the legendary Achilles, Patroclus dies in battle after being stabbed with a spear by the Trojan Hector. When Achilles hears this news, he is overcome with grief, and he goes into a frenzy of anger, guilt, and vengeance. He immediately returns to battle, and he kills so many Trojans that the river is clogged with their dead bodies. He then confronts Hector. Hector wants to flee, knowing he is no match for Achilles, but the gods, deciding it is Hector's time to die, convince him to stay. Achilles avenges Patroclus's death by killing Hector and then, still in a mad frenzy, dishonors his corpse. Rather than returning it to Tory or giving it a proper burial, Achilles ties Hector's body to the back of his chariot and rides three times around the walls of Troy, dragging the body.
Achilles's reaction to Patroclus's death shows how deeply he cared about his friend and how he grieves his parting. We also see how powerful a warrior Achilles is once he is reengaged in fighting.